1. Field of Endeavor
The present disclosure relates to a method for compensating instantaneous power failure in medium voltage inverter and a medium voltage inverter system by using the same.
2. Background
This section provides background information related to the present disclosure which is not necessarily prior art.
In general, an inverter interrupts a PWM (pulse width modulation) output within several ms if a power failure occurs in an input power. At this time, if a load is large in inertia, it takes a long time to accelerate the load when the power is restored. This type of operation may result in a great loss in an industrial site, and if an inverter is stopped, an instantaneous power failure compensating technique for the inverter is applied to an industrial site where a great loss is expected due to process failure.
FIGS. 1a and 1b illustrate an operation of an instantaneous power failure compensating apparatus in an inverter according to prior art, where FIG. 1a illustrates the apparatus in a normal state, while FIG. 1b illustrates the apparatus when a power is interrupted.
In general, an electrolytic condenser (210) embedded in an inverter (200) (the condenser is illustrated outside of the inverter for convenience of explanation) is charged with a power from the inverter (200) in a normal state (FIG. 1a), and a load (300) is driven (FIG. 1b) using the power charged in the condenser (210), in a case the power (100) is interrupted by power failure. At this time, the inverter (200) can drive the load (300) without any interruption, because capacity of the conventional electrolytic condenser (210) is so designed as to normally operate if an instantaneous power failure is within 16 msec. However, the inverter (200) may disadvantageously stop operating in an area where power condition is not good and power failure lasts more than 16 msec, thereby creating a great loss to an industrial site.
Meanwhile, the current trend is that demand for energy saving and medium voltage inverter grows larger. As an inverter that satisfies the demand, an inverter having a cascade H-bridge (hereinafter referred to as ‘CHB’) structure is largely used. The inverter of CHB structure, being largely used for essential facilities at industrial sites, needs reliability as an important factor.
However, the conventional instantaneous power failure compensating apparatus as shown in FIG. 1a suffers from a disadvantage of failing to overcome the instantaneous power failure if applied with an inverter having a CHB structure, the reason of which may be summarized as below.
First, the conventional instantaneous power failure compensating apparatus cannot control a DC-link of a plurality of unit power cells in an inverter. Second, the conventional instantaneous power failure compensating apparatus uses a feedback reference voltage as a voltage command of the DC-link, and in a case the feedback reference voltage is actually applied in the medium voltage inverter, a DC-link voltage of each power cell comes to differ due to parasitic elements of capacitors, and therefore it is impossible to drive the inverter using one voltage command during actual operation. Third and lastly, the conventional instantaneous power failure compensating apparatus fails to provide a solution in consideration of external environment involving a large scale of load mounted with a medium voltage inverter having a CHB structure.
Thus, there is a need to provide a medium voltage inverter capable of solving the aforementioned disadvantages or problems.